Blasting agent for blasting in hot boreholes

ABSTRACT

A blasting agent resistant to decomposition when exposed to elevated temperatures, e.g., 325* - 350* F. for 24 hours, comprises an inorganic oxidizing salt such as ammonium nitrate; a high-boiling liquid oxygen-containing organic fuel, e.g., dibutyl phthalate; a gelling agent, preferably an acrylamide polymer; and preferably a densifying agent such as ferrophosphorus and a coating agent such as calcium stearate. A primer assembly containing the thermally stable blasting agent in a cartridge can having a well at one end containing a high-explosive booster attached to high-energy detonating cord, which is in initiating relationship with a blasting cap. The metal-cartridged blasting agent and primer assembly are useful for blasting in hot boreholes, which can be either wet or dry.

nited States Patent 1191 Schaefer BLASTING AGENT FOR BLASTING IN HOT BOREHOLES [75] Inventor: William E. Schaefer, l-lagerstown,

[73] Assignee: E.I. du Pont de Nemours and Company, Wilmington, Del.

[22] Filed: Jan. 24, 1973 [21] Appl. No.: 326,378

[52] US. Cl 149/21, 149/41, 149/43, 149/44, 149/60, 149/61 [51] llnt. Cl C06b 1/04 [58] Field of Search 149/46, 60, 61, 43, 44, 149/41, 21

[56] References Cited UNITED STATES PATENTS 3,622,408 11/1971 Lyerly 149/43 x 3,640,784 2/1972 Yancik et al. 149/43 111] 3,819,429 1 June 25, 1974 Primary Examiner-Stephen J. Lechert, Jr.

[57] ABSTRACT A blasting agent resistant to decomposition when exposed to elevated temperatures, e.g., 325 350 F. for 24 hours, comprises an inorganic oxidizing salt such as ammonium nitrate; a high-boiling liquid oxygen-containing organic fuel, e.g., dibutyl phthalate; a gelling agent, preferably an acrylamide polymer; and preferably a densifying agent such as ferrophosphorus and a coating agent such as calcium stearate. A primer assembly containing the thermally stable blasting agent in a cartridge can having a well at one end containing a high-explosive booster attached to highenergy detonating cord, which is in initiating relationship with a blasting cap. The metal-cartridged blasting agent and primer assembly are useful for blasting in hot boreholes, which can be either wet or dry.

9 Claims, 1 Drawing Figure BLASTING AGENT FOR BLASTING IN HOT BOREHOLES BACKGROUND OF THE INVENTION The present invention relates to blasting compositions known as nitro-carbo-nitrates especially adapted for use in blasting where elevated temperatures, e.g., temperatures of several hundred degrees, may be encounteredin the boreholes into which the compositions are to be loaded.

It sometimes is necessary to conduct blasting operations in drill holes in hot rock formations such as those which overlie burning coal mines or natural underground heat sources. In the past, hot hole" blasting was accomplished successfully, for example, by the use of a nitro-carbo-nitrate composition based on ammonium nitrate sensitized with dinitro-toluene. The term nitro-carbo-nitrate is used in the blasting art to denote compositions, usually ammonium nitrate sensitized with nitro bodies, which contain no self-explosive ingredient and are themselves insensitive to initiation by a single commercial detonator of ordinary strength. The nitro-carbo-nitrate composition which was used previously in hot hole blasting was packed into metal cans, which were loaded into the boreholes and detonated by means of a similarly canned primer composition, the latter comprising a high explosive sensitive to initiation by high-energy detonating cord. Although the nitro-carbo-nitrate blasting agent exhibited satisfactory thermal stability for blasting in hot boreholes, it suffered from the disadvantage that its preparation involved a costly hot-mixing procedure. Furthermore, although the blasting agent contained no high explosive and therefore was not classed as such, a high explosive was loaded into the primer cans and sealed therein, and therefore these cans had to be stored, transported, and otherwise handled as high explosives. An additional disadvantage associated with the primer and the blasting assembly containing it was that the primer can had to be sheathed in an insulating material such as wet sand or silt held in an outer container to assure that the more heat-sensitive high explosive in the primer can was sufficiently protected from the heat of the borehole.

SUMMARY OF THE INVENTION This invention provides a blasting agent adapted for use in hot boreholes comprising, in mixture, by weight, about from (a) 70 to 97% of an inorganic oxidizing salt selected from the group consisting of ammonium nitrate, sodium nitrate, and mixtures thereof; (b) 1.0 to 10.0% of a liquid oxygen-containing organic fuel which boils at a temperature above about 400 F., preferably an aliphatic or aromatic polyester, e.g., dibutyl phthalate; and (c) 0.1 to 3.0% of a gelling agent selected from the group consisting of acrylamide polymers, galacto mannans, and mixtures thereof. Preferably, the blasting agent also contains up to about by weight of a densifying agent selected from the group consisting of ferrophosphorus, ferrosilicon, iron oxide, silicon carbide, and mixtures thereof and up to about 3.0% by weight of a coating agent such as calcium stearate, which, in addition to serving as a packing aid for achieving high density, has a beneficial effect on the behavior of the mixture at elevated temperatures.

Also provided by this invention is a primer assembly adapted to effect the detonation of blasting agents in hot boreholes comprising (a) a cylindrical metal container closed at both ends and having a recess at one end thereof, the container enclosing the blasting agent of the invention described above; and (b) a charge of secondary high explosive, preferably RDX (cyclotrimethylenetrinitramine), within the recess, the charge being attached to a length of high-energy detonating cord, e.g., by threading the cord through the aperture of one or more annular disks of explosive and causing the disk(s) to rest upon a knotted end of the cord, and the charge being separated from the mouth of the recess by a layer of thermally insulating material, e.g., an earthen material such as wet sand or silt, which confines the charge within the recess as well as any portion of the length of detonating cord located in the recess, a portion of the cord being located outside the recess and leading to a blasting cap. To prevent the cord and charge from being inadvertently pulled out of the recess, the length of cord attached to the charge of high explosive may be attached to another length of cord, i.e., a downline, which has one end attached to the container, e.g., knotted to a handle or bail affixed to the end of the container having the recess, and the other end leading to a blasting cap, the lengths of cord being attached together in a manner such that the length of cord attached to the explosive charge is unresponsive to tension on the downline.

In contrast to the prior art compositions, the blasting agent of the invention can be made by a relatively simple cold-mix process and, by virtue of its water resistance, can be used in cans which have been perforated to permit the release of vapors formed as a result of the heat in the borehole. The primer assembly affords the advantage that the parts of the assembly can be put together in the field just prior to loading into the borehole, and therefore the primer container, containing no high explosive, can be shipped, stored, and otherwise handled as a nitro-carbo-nitrate, as are the remaining containers of blasting agent which the primer is intended to detonate. Furthermore, because of the high degree of thermal stability of the blasting agent employed in the primer container, no insulation or further containment of the primer is required.

BRIEF DESCRIPTION OF THE DRAWING The primer assembly of the invention will be described with reference to the attached drawing which is a longitudinal cross-sectional view of one embodiment thereof.

DETAILED DESCRIPTION The blasting agent of the present invention is characterized by a high degree of thermal stability, undergoing, when exposed to a temperature of about 325 350 F. for 24 hours, no rapid decomposition, e.g., explosion, combustion or fume-off, or any other form of decomposition which renders it insensitive to initiation by the detonation of a high-explosive booster. Thus, when the blasting agent is said to be resistant to decomposition when exposed to a temperature of about 325 350 F. for 24 hours," it is meant that the composition does not explode, burn, or fume-off as a result of such exposure, and that after such exposure it can still be initiated reliably by means of a booster. The term high-explosive booster is employed in the con 3 ventional sense, denoting a charge of secondary high explosive which reinforces the detonation wave from a detonator or detonating cord.

In addition to having high thermal stability, the present blasting agent, which is granular, is resistant to, i.e., does not become desensitized by, the presence of water. This characteristic is advantageous for hot hole blasting in wet boreholes inasmuch as the metal cans in which the agent is packed may be vented so as to permit the release of any vapors formed as a result of the heat in the borehole. In some cases wetting of the boreholes may be necessary if the borehole temperature is excessively high.

In addition to the oxidizing component of the mixture, i.e., ammonium nitrate, sodium nitrate, or a mixture of these two salts, the present composition contains a high-boiling liquid fuel component, i.e., an oxygen-containing organic fuel, such fuels, as a rule, having a greater degree of thermal stability than others. Use of a fuel which is a liquid at room temperature permits the composition to be made with standard coldmix equipment, thereby eliminating the need for special hot-mixing apparatus. At the same time, the liquid fuel is high-boiling, i.e., boils at a temperature above about 400 F. at atmospheric pressure, thereby assuring a high flash point and the required thermal stability of the composition. Although any liquid oxygencontaining organic fuel having the required high boiling point can be employed, e.g., esters, ethers, ketones, etc., aliphatic or aromatic polyesters, i.e., compounds having two or more ester groups, are preferred on the basis of suitability as well as economics, Examples of such esters which can be employed in the present composition are esters of phthalic acid such as dibutyl phthalate, sebacic acid such as dibutyl sebacate, and butyric acid such as glyceryl tributyrate.

The amounts of ammonium nitrate and/or sodium nitrate, and liquid fuel in the composition can vary, but generally sufficient fuel is employed so that the total composition has an oxygen balance of about from 25 to +15%, and preferably about from 10 to +10%, consideration being given also to the fuel value which may be contributed by a densifying agent which may be employed, ferrophosphorus and ferrosilicon both being heavy metallic fuels. A densifying agent preferably is employed to facilitate the sinking of the canned composition in boreholes to which water has been added. Iron oxides also can be used for this purpose, although fuel densifiers, particularly ferrophosphorus, are preferred inasmuch as they contribute to the energy of the detonation and the desired oxygen balance is more readily achieved therewith.

In general, the inorganic oxidizing salt is present in the amount of about from 70 to 97%, and preferably 80 to 90%; liguid fuel 1.0 to 10, and preferably 1.0 to 5%; and densifying agent, if present, 1.0 to 20%, by weight of the mixture. The amount of liquid fuel used needs to be sufficient so as to be uniformly dispersed throughout the mixture, as well as to provide the desired oxygen balance and sensitivity to initiation. On the other hand, the liquid fuel content does not exceed about and preferably about 5%, by weight inasmuch as the sensitivity of the mixture to initiation becomes reduced at higher levels. The liquid fuel becomes dispersed as a result of the cold mixing process employed to make the composition, and the final composition is granular, generally having a packing denity of about 1.05 g/cc or 4 higher, such densities being desirable from the standpoint of energy production as well as sinkability in wet boreholes. In the preferred blasting agent of the invention, the inorganic oxidizing salt is comprised of at least about by weight of ammonium nitrate.

The particle size of the inorganic oxidizing salt can vary, and a mixture of coarse and fine sizes often will be advantageous for purposes of achieving higher packing density. A blend of ammonium nitrate prills with smaller ammonium nitrate particles, e.g., about a 50/50 blend of whole and crushed prills, has been found to be particularly useful. Prills generally are larger than 20 mesh, and smaller than 8 mesh, in size.

The particle size of the densifying agent also can vary, usually being selected on the basis of a size which affords a desired density and good dispersion. Preferably, the densifier particle size is less than the inorganic oxidizing salt particle size, or the coarsest size in blends, e.g., about one-fifth to one-tenth, or less, the prill size when prills are employed.

The blasting agent of the invention is packed in metal cans and sealed therein. However, at the time the cans are loaded into the boreholes, if the borehole temperature is high enough, e.g., above about 350 F that the cans could rupture due to internal pressure buildup, the cans are punctured to release the pressure. For example, one or more holes about 5 1 to k inch in diameter may be punched in the side wall of the can. The presence of water in the borehole does not deleteriously affect the present blasting agent in perforated cans inasmuch as the composition contains a gelling agent, i.e., an acrylamide polymer, a galactomannan such as guar gum, or a mixture of these, which imparts waterresistance to the composition. As a rule, about from 0.1 to 3.0% of the gelling agent is sufficient to form a protective gel with any water which may enter the can through the perforations. The galactomannan or acrylamide polymer can be cross-linked in the well-known manner, or partially hydrolyzed acrylamide polymer can be employed. It has been found that blasting compositions ofthis invention which contain polyacrylamide are superior with respect to thermal stability, and such compositions therefore are preferred over those containing only galactomannan gelling agents, in cases in which a maximum degree of stability is required.

Although not required in the present blasting agent, the incorporation of a small amount, e.g., about from 0.1 to 3.0% of calcium stearate in the composition assists in affording a higher packing density and has an additional beneficial effect on the high-temperature characteristics of the composition, i.e., prevents the occurrence of foaming.

As was stated previously, the blasting agent of this invention can be manufactured by a relatively simple cold-mix procedure, using standard feeding and mixing equipment. For example, the solid ingredients and the liquid fuel can be metered into an auger and mixed together therein as they move toward a product hopper. For cartridging, the mixture can be auger-fed from the hopper into metal cans, after which the metal closures are placed on the open ends and the cans sealed. The mixture is packed into the cans as tightly as practical to assure maximum water resistance in case the cans are to be punctured in use.

The cans of blasting agent, after loading into the boreholes, are initiated by the detonation of a primer can, which is the outermost can in the column. The primer can is part of the primer assembly of this invention, which now will be described with reference to the accompanying drawing. in the drawing, a cylindrical metal container 1, e.g., made of steel, referred to as a can, is closed at both ends and has a concavity, recess, or well 2 formed in one of its end surfaces. This container, together with the granular high-temperature-stable, temperature-stable, water-resistant blasting agent 11 of this invention packed and sealed therein, form a primer can. Because the primer can, like the other cans of blasting agent, contains no high explosive, it can be handled as a nitro-carbo-nitrate for storage and transportation purposes. The remainder of the of the primer assembly, which is joined to the primer can at the blasting site, includes a booster charge of secondary explosive, e.g., pressed RDX, 3, the charge in this case being in the form of two annular pellets; a first length of high-energy detonating cord, e.g., Primacord," 4, which is threaded through the apertures in the pellets, the latter being seated against a knot 5 at one end of cord 4; and a second length of high-energy detonating cord 6, which is attached at one of its ends, by a knot 7, to a bail or handle 8 affixed to the end surface of can 1. Cord 6 is known as a downline, and is used for connecting the primer assembly to a trunlcline and blasting cap system. The other end of cord 6 is in initiating relationship with a blasting cap (not shown).

Charge 3 is lowered into well 2 in a manner such that knot 5 rests on the bottom of well 2 and cord d is substantially coaxial with well 2, the other end of cord 4 being attached to downline 6 by knot 9, formed just above knot 7, so that any tension or pull on cord 6 will not be able to pull cord 4 out of well 2. After booster charge 3 has been positioned within well 2, a thermally insulating material 10, such as wet sand or silt, is packed into the space around charge 3 and cord 4 to the mouth of well 2, thereby serving to protect these components of the assembly from the heat to be encountered in the borehole.

Before the cartridged blasting composition and primer assembly of the invention are to be employed in hot hole blasting, e.g, in holes at about 150 F. or above, the highest temperature in each borehole preferably should be determined shortly before loading of the borehole is to begin. The time the composition can be kept in the borehole before firing depends on this temperature, the maximum permissible exposure time being greater with lower borehole temperatures. For example, with borehole temperatures above about 350 F the exposure time to the heat preferably is no more than about 10 minutes, whereas an exposure time on the order of a half hour can be permitted with borehole temperatures of about 150 200 F. Although the blasting agent per se is stable after exposure to about 325 350 F. for 24 hours, much shorter exposure times are recommended for field use where a safety margin must be allowed for errors in temperature measurement, and variations in borehole temperature with time and depth. In addition, the greater heat sensitivity of the components of the primer assembly needs to be considered. RDX is a preferred booster charge because it is one of the most heat-stable of the commonly available secondary explosives, and in some instances it likewise may be preferred to use detonating cord having an RDX core. in any event, it is good practice to direct a full stream of water into boreholes whose temperature is above 200 F. during the hole loading and stemming operation, and not to load boreholes at 450 F. or higher until the temperature has been lowered by EXAMPLE 1 The following materials are mixed together in amounts sufficient to provide the indicated number of parts per 100 parts of mixture by weight: 82.5 parts of a blend of whole and crushed ammonium nitrate prills such that 50 60% of the particles pass through a 20 mesh screen, and 13 23% pass through a IOO-mesh screen (Tyler), 13 parts of mesh ferrophosphorus, 3.0 parts of dibutyl phthalate, 1.0 part of polyacrylamide, and 0.5 part of calcium stearate. The polyacrylamide is the sodium salt of a copolymer of acrylamide and acrylic acid having a molecular weight of 13 15 million, sodium content of 5.2%, and a particle size such that 98% of the particles pass through a 20- mesh screen. The blasting agent mixture is auger-fed from a hopper into steel cartridge cans 24 inches long and 6.5 inches in diameter, the packing density being 1.20 g/cc. The weight of mixture in each can is a nominal 36 pounds. A can end is seamed into place at the open end of each can, one end of each can having a bail afiixed thereto. Some of the cans have one end which is fitted with a well 3.7 inches long and 1.7 inches in diameter. These cans are primer cans and contain a nominal 35.5 pounds of the mixture.

The cartridged blasting agent described above is loaded into two hot boreholes, each hole being approximately 40 feet deep and 9% inches is diameter. The initial temperature of the holes is 650 F., and the holes are flushed with water for 15 minutes before loading and during loading and stemming. Soaking reduces the borehole temperature to about 350 F. or less before loading begins. One of the holes is loaded with six cans and the other with eight cans, all cans having been punctured in their side walls with three %-inchdiameter holes to provide venting. The outermost can in each borehole is a primer can the well of which contains, as a booster charge, two 35-gram pellets (each l-inch thick and 1.5 inches in diameter) of pressed RDX strung on a length of Primacord as shown in the drawing, which in turn is knotted to a Primacord" downline, one end of which is knotted to the bail on the primer can. The space in the well around the booster charge and the Primacord is packed with wet sand, which material is also used to produce a stemming of about 25 feet over the blasting agent column. The downlines from the holes are initiated with an electric blasting cap nine minutes after loading of the holes has begun. Both holes shoot satisfactorily, as evidenced by the excellent heave of the formation.

EXAMPLE 2 a. A 25-gram sample of the blasting agent mixture described in Example 1 is placed in a Pyrex test tube containing a thermocouple in a glass well. The test tube is placed in a triethylene glycol bath and heated at 375 7 F. for 24 hours. The sample does not explode, burn, fume off, or exhibit a temperature rise above the bath temperature.

b. A 25-gram sample of the blasting agent mixture described in Example 1, but from which the calcium stearate has been omitted, subjected to the conditions described in Paragraph (a) above, likewise does not explode, burn, fume off, or exhibit internal heating above the bath temperature, but the sample foams under the described conitions.

EXAMPLE 3 V The blasting agent mixture (10.7 pounds) described in Example 1 is packed into a l-gallon can, which is pierced with two %-inch-diameter holes in its side wall. The can is soaked for 20 minutes in water at 140 F. under a pressure of 25 psi. Two 35-gram pressed RDX pellets on a length of Primacord are then inserted into the water-soaked product. When the Primacord" is initiated with an electric blasting cap, the blasting agent detonates.

I claim:

1. A blasting agent for use in hot boreholes comprising, in mixture, by weight, about from a. 70 to 97% of an inorganic oxidizing salt selected from the group consisting of ammonium nitrate, sodium nitrate, and mixtures thereof;

b. 1.0 to 10% of a liquid ester which boils at a temperature above about 400 F. at atmospheric pressure; and

c. 0.1 to 3.0% of a gelling agent selected from the group consisting of acrylamide polymers, galactomannans, and mixtures thereof.

2. A blasting agent of claim 1 containing up to about 20% by weight of a densifying agent selected from the group consisting of ferrophosphorus, ferrosilicon, iron oxide, silicon carbide, and mixtures thereof.

3. A blasting agent of claim 1 containing up to about 3% by weight of calcium stearate.

4. A blasting agent of claim 1 wherein said inorganic oxidizing salt is ammonium nitrate.

5. A blasting agent of claim 1 wherein said inorganic oxidizing salt is a mixture of coarse and fine particles.

6. A blasting agent of claim l'wherein said est'eris se-' lected from the group consisting of aromatic and aliphatic polyesters.

7. A blasting agent of claim 6 wherein said polyester is dibutyl phthalate.

8. A blasting agent which is resistant to decomposition at elevated temperatures and to desensitization by exposure to water comprising, in mixture, by weight, about from a. to 97% ammonium nitrate;

b. 1.0 to 20% of a densifying agent selected from the group consisting of ferrophosphorus and ferrosilicon;

c. 1.0 to 10% of dibutyl phthalate;

' d. 0.1 to 3.0% of acrylamide polymer; and

e. 0.1 to 3.0% of calcium stearate. 9. A blasting agent of claim 8 wherein said ammonium nitrate is a mixture of coarse and fine particles. 1 

2. A blasting agent of claim 1 containing up to about 20% by weight of a densifying agent selected from the group consisting of ferrophosphorus, ferrosilicon, iron oxide, silicon carbide, and mixtures thereof.
 3. A blasting agent of claim 1 containing up to about 3% by weight of calcium stearate.
 4. A blasting agent of claim 1 wherein said inorganic oxidizing salt is ammonium nitrate.
 5. A blasting agent of claim 1 wherein said inorganic oxidizing salt is a mixture of coarse and fine particles.
 6. A blasting agent of claim 1 wherein said ester is selected from the group consisting of aromatic and aliphatic polyesters.
 7. A blasting agent of claim 6 wherein said polyester is dibutyl phthalate.
 8. A blasting agent which is resistant to decomposition at elevated temperatures and to desensitization by exposure to water comprising, in mixture, by weight, about from a. 70 to 97% ammonium nitrate; b. 1.0 to 20% of a densifying agent selected from the group consisting of ferrophosphorus and ferrosilicon; c. 1.0 to 10% of dibutyl phthalate; d. 0.1 to 3.0% of acrylamide polymer; and e. 0.1 to 3.0% of calcium stearate.
 9. A blasting agent of claim 8 wherein said ammonium nitrate is a mixture of coarse and fine particles. 